Kinetics of Suspended Emulsion Copolymerization of Acrylonitrile with Itaconic Acid

http://www.e-polymers.org e-Polymers 2008, no. 037 ISSN 1618-7229

Kinetics of suspended emulsion copolymerization of acrylonitrile with itaconic acid

Yu Mengmeng, Chen Hou,* Liang Ying, Cui Hengli, Zhou Wenying, Li Dongmei, Cui Xianqiang

School of chemistry and Materials Science, ludong University, 264025 Yantai, China; fax: +865356697667; e-mail: [email protected]

(Received: 20 September, 2008; published: 07 April, 2009)

Abstract: Acrylonitrile(AN) was successfully used to copolymerize with itaconic acid (IA) by suspension emulsion polymerization at 70 ˚C under N2 atmosphere for the first time. Porous AN/IA copolymers were prepared by using potassium peroxydisulphate (KPS) as initiator, poly(vinyl alcohol) (PVA) as dispersant and Span60 as emulsifier. Kinetics of suspension emulsion copolymerization of AN with IA were studied. Effects of various mass ratios of water/monomer, initiator concentrations, emulsifier concentrations and dispersant concentrations on AN/IA copolymerization rate and changes of particle size and size distribution with the extension of polymerization time were investigated. It was found that the copolymerization rate increased with water/monomer mass ratio and KPS concentration. Span60 concentration and PVA concentration had no obvious effect on the polymerization rate. Finally, a suspension emulsion polymerization mechanism for AN and IA was proposed. Keywords: Suspension emulsion polymerization, Kinetics, Acrylonitrile, Itaconic acid

Introduction Polyacrylonitrile(PAN) resins plays an important role in various industrial applications. For example, PAN was used as reinforced material of concrete and precursors of carbon fibers [1], and also used to support catalyst in polyketone synthesis [2]. In addition, super absorbent resins prepared by PAN resins has many advantages such as high water absorption and good water retention, and are widely applied in medical, health, agriculture and horticulture [3]. Copolymers of AN/IA have found a wider range of applications due to their chemical and thermal stability and mechanical and optical properties on comparing with homopolymer of AN [4-12]. Porous resins have many applications in the coatings, ink, cosmetics, and paper industries, mainly because of their potential ability to easily remove unreacted monomer, protect against ultraviolet radiation and to manipulate the difference in the refractive index between the polymer and air [13]. Besides, Porous polymers are beneficial to the immersion of thermal and solvent and could be processed under mild conditions. It is important to select a suitable technique and optimize the conditions of polymerization for synthesis of easy processable AN copolymers. AN copolymers synthesized by traditional methods, such as solution, aqueous suspension and solvent water suspension techniques had some defects in its compact structure and difficulty in processability [14-19]. Vindevoghel et al proposed suspension emulsion polymerization process and applied this process to prepare porous poly(vinyl chloride) (PVC) particles [20, 21]. Suspension emulsion

1 polymerization is a new and promising technique that may be used in the production of porous and high-performance polymers, which cleverly combines the advantages of suspension polymerization and those of emulsion polymerization. Bao et al investigated the influence of conditions on particle characteristic of PVC resin and poly(methyl methacrylate) (PMMA) prepared by suspension emulsion polymerization [22-24]. To the best of our knowledge, there are no reports on suspension emulsion copolymerization of AN with IA except our group. We have successfully prepared porous AN/IA copolymers by suspension emulsion polymerization, and studied effects of polymerization conditions on structures and properties of AN/IA copolymers. Kinetics of suspension emulsion copolymerization of AN with IA was first time attempted in the present work. In this paper, polymerization kinetics of AN/IA copolymers prepared by suspension emulsion polymerization were investigated for the first time; a detailed study for effects of KPS concentration, mass ratio of water/monomer, Span60 concentration and PVA concentration on polymerization kinetics of AN/IA copolymers were described. Finally, a suspension emulsion polymerization mechanism for AN and IA was proposed.

Results and discussion In this paper, AN/IA copolymers were successfully prepared by suspension emulsion polymerization. Figure 1 shows SEM microphotograph of AN/IA copolymer particles prepared by suspension emulsion polymerization according to the Standard Recipe in Tab.1. As shown in Figure 1, the AN/IA copolymer particles had high porosity.

Fig. 1. SEM microphotograph of AN/IA copolymer particles prepared by suspension emulsion polymerization.

Effect of Initiator Concentration KPS was used as the initiator in copolymerization system of AN/IA; polymerization kinetic curves of various initiator KPS concentrations based on the weight of AN are shown in Figure 2. The polymerization conditions were kept constant as Tab.1 except KPS. As shown in Figure 2, the polymerization rate increased significantly with increase in KPS concentration. Upon increasing KPS concentration, the production rate of free radicals increased the number of active sites and growing chains

2 increased, which led to the number of latex particles increasing, the probabiliies of the latex particles colliding with each other and forming primary particles augmented, more primary particles formed porous particles and the polymerization rate increased.

Conversion/% 10

0 45 90 135 180 Polymerization time/min

Fig. 2. Polymerization kinetic curves of various initiator KPS concentrations. (■) 0.64 wt%; (●) 0.74 wt%; (▲) 0.82 wt%.

Initiator concentration also influences the number and the diameters of latex particles, and the particle size and size distribution of resins. When KPS concentration is 0.74 wt%, the particle size and size distribution of AN/IA copolymers with different polymerization time is shown in Figure 3.

0.5 h 8 1.5 h 3.0 h 6

Differential volume/% 0 0 500 1000 1500 2000 Particle size/ m

Fig. 3. Particle size and size distribution of AN/IA copolymers with different polymerization times.

As shown in Figure 3, the particle size increased and size distribution became wide with prolonged polymerization time. After the formation of the latex particles, initiator

3 decomposed continuously and a large number of free radical still produced and grew in the water phase, the growing free radicals could form new latex particles reaching the critical chain length, or absorbed by the existed particles before reaching the critical chain length. The latex particles could coagulate and form new primary particles, or coalesce with particles. The above process continued throughout the polymerization, even in later period of polymerization, the new formed free radicals still initiated copolymerization of AN with IA, formed primary particles and then agglomerated large and non-uniform particles. As the result, the size distribution became wide.

Effect of Mass Ratio of Water/Monomer Water acted as the dispersed phase in suspension emulsion polymerization, the initiator KPS dissolved in the water and initiated the polymerization of AN with IA. Figure 4 displays polymerization kinetic curves of different mass ratios of water/monomer. The polymerization conditions were kept constant as Tab.1 except the weight of water. As shown in Figure 4, the polymerization rate and the final conversion increased with mass ratio of water/monomer increase. Upon increasing mass ratio of water/monomer, monomer and KPS dissolved easily in the water phase, the decomposition efficiency of KPS increased and then the free radicals in the water phase, active sites, growing chains and the number of latex particles increased, the probabiliies of the latex particles colliding each other and forming primary particles increased, more primary particles formed porous particles and the polymerization rate became higher. 50

Conversion/% 10

0 45 90 135 180 Polymerization time/min

Fig. 4. Polymerization kinetic curves of different mass ratios of water/monomer. (■) 0.2:1; (●) 0.3:1; (▲) 1.0:1.

Coplymerization of AN/IA mainly proceeded in the dispersed water droplets through the emulsion polymerization mechanism because of the solubility of monomer in the water and the application of the water-soluble initiator. When mass ratio of water/monomer is 0.2:1, the particle size and size distribution of AN/IA copolymers with different polymerization time is shown in Figure 5. As shown in Figure 5, the particle size increased and size distribution became narrow with prolonged polymerization time. Because polymerization occurred in the water phase, with copolymerization of AN/IA proceeding, more monomer diffused into water droplets

4 which had low monomer concentration and polymerized by suspension emulsion polymerization. With the extension of polymerization time, the particle size increased and became uniform, thus the size distribution became narrow.

10.0 0.5 h 1.5 h 7.5 3.0 h

5.0

2.5 Differential volume/% 0.0 0 500 1000 1500 2000 Particle size/ m

Fig. 5. Particle size and size distribution of AN/IA copolymers with different polymerization times.

Effect of Dispersant Concentration PVA was used as dispersant in suspension emulsion copolymerization system of AN/IA. Polymerization kinetic curves of various dispersant PVA concentrations based on the weight of AN is illustrated in Figure 6.

20 Conversion/% 10

0 45 90 135 180 Polymerization time/min

Fig. 6. Polymerization kinetic curves of various dispersant PVA concentrations. (■) 0.11 wt%; (●) 0.18 wt%; (▲) 0.25 wt%.

The polymerization conditions were kept constant as Tab.1 except PVA. Upon increasing PVA concentration, the changes of polymerization rate became less prominent. In the prior period of polymerization, the polymerization rate and the final conversion increased faster with PVA concentration at 0.18 wt% than at 0.11 wt% in the middle and later periods of polymerization. However, the continuous increasing of PVA concentration had no obvious effect on the polymerization rate. PVA had amphipathic structure, lipophilic polymer chain adsorbed on the surface of monomer

5 phase, hydrophilic group orientated to the water phase. Because the water was dispersed into monomer phase, a protective film was constituted on the surface of water droplet. Therefore, PVA had a protective effect on liquid droplets. Because the stability of liquid droplets increased, more monomer diffused into water phase and the polymerization rate increased with dispersant concentration increasing. But more PVA could increase the viscosity of water phase, which went against the diffusion rate of monomer. Therefore, it hindered the increase of polymerization rate. Dispersant PVA was soluble in the dispersed phase in suspension emulsion polymerization. When PVA concentration is 0.248 wt %, the particle size and size distribution of AN/IA copolymers with different polymerization time is shown in Figure 7. As shown in Figure 7, the particle size increased and size distribution became wide along with the polymerization time. PVA increased the viscosity of liquid phase, hindered the diffusion of monomer, the monomers could not diffuse from the oil phase to the aqueous phase with prolonged polymerization time. The particle size increased, and the particle became non-uniform. As a result, the size distribution became wide.

6.5 0.5 h /% 5.2 1.5 h 3.0 h

3.9

2.6

1.3 Differential volulme 0.0 0 500 1000 1500 2000 Particle size/ m

Fig. 7. Particle size and size distribution of AN/IA copolymers with different polymerization times.

Effect of Emulsifier Concentration In suspension emulsion polymerization, a part of the emulsifier would be present not only at the polymer particle/water interface but at the monomer/water interface on comparing with a conventional emulsion polymerization. Another point is that in most cases the amount of the emulsifier is very low, so that the particles formed by the emulsion process are not stable enough and easily coalesce to give much larger primary particles [20, 21]. A set of experiments had been carried out using Span60 as emulsifier in suspended emulsion copolymerization of AN with IA. Figure 8 displays polymerization kinetic curves of various emulsifier Span60 concentrations based on the weight of AN. The polymerization conditions were kept constant as Tab.1 except Span60. As shown in Figure 8, Span60 concentration had no obvious effects on the polymerization rate. Emulsifier Span60 had only colloid protection behavior on liquid droplets and latex particles, and the amount of Span60 was very low, the emulsifier had little effect on the total polymerization process, thus the change of polymerization rate was not significant with increasing Span 60 concentration.

20 Conversation/% 10

0 45 90 135 180 Polymerization time/min

Fig. 8. Polymerization kinetic curves of various emulsifier Span60 concentrations. (■) 0.60 wt%; (●) 0.67 wt%; (▲) 0.70 wt%.

Emulsifier Span60 could not only reduce interface tension of liquid droplets, but also had emulsification and dispersing effects on the liquid phase. When Span60 concentration is 0.6 wt %, the particle size and size distribution of AN/IA copolymers with different polymerization time is shown in Figure 9. As shown in Figure 9, the particle size increased and size distribution became narrow along with the extension of polymerization time. When emulsifier Span60 was present, the latex particles were more stable, the agglomerating probabilities of the latex particles decreased; as copolymerization of AN/IA proceeded, the particle size became more uniform. Thus, the size distribution became narrow.

10.0 0.5 h 1.5 h 7.5 3.0 h

5.0

2.5 Differential volume/% 0.0 0 500 1000 1500 2000 Particle size/ m

Fig. 9. Particle size and size distribution of AN/IA copolymers with different polymerization times.

Mechanism of forming porous AN/IA copolymers particles for the suspended emulsion Figure 10 presents a schematic procedure for the porous particles nucleation and particle growth of the AN/IA copolymers with suspension emulsion polymerization. At

7 the beginning of the process, the aqueous phase in the shape of water droplets dispersed in the continuous monomer phase, water-soluble initiator KPS discomposed and formed free radicals in the aqueous phase, and partial monomers had definitely some solubility in water phase, so the polymerization of AN/IA was carried out in aqueous phase initially. Partial free radicals initiated monomers to polymerize in the aqueous phase, along with the progress of chain growth, the solubility of free radicals chains in the water became poor. When reaching the critical chain length, free radicals chains curled and entwisted in themselves and settled out from aqueous phase, and then formed initial particles. A little bit free radicals diffused into the monomer phase and initiated the polymerization of monomer. Because AN/IA copolymers was not soluble in monomer, free radical chains in monomer phase settled out, and then coagulated with particles formed in the aqueous phase. As a result, the primary particles formed. The primary particles polymerized with monomers dissolved in the aqueous phase and grew sequentially. Along with the progress of polymerization, the monomers in water phase were consumed gradually, and then the monomers diffused from the oil phase into the aqueous phase, so that the polymerization could continue. Finally, the primary particles aggregated into the proposed porous particles.

Aqueous Coagulating phase Initial particle Aqueous R•+AN+IA phase Curly free Primary Coagulatin

radical monomer pariticleaggregating phase Initial particleg

Porous particle

Fig. 10. Mechanism of forming porous AN/IA copolymers particles with suspension emulsion polymerization.

Conclusions Kinetic study on suspended emulsion copolymerization of AN with IA showed that the rate of copolymerization increased with the increase of KPS concentration and water/monomer mass ratio. Span60 concentration and PVA concentration had no obvious effect on the polymerization rate. The copolymerization system of AN/IA existed continuously in different state changes with increase in conversion, from initial liquid phase to powdery particles; water was encapsulated by the particles

8 swollen by monomer. When the conversion reached 40%, it was difficult to keep initial agitation rate. Thus the conversion of suspension emulsion polymerization of AN/IA was controlled in a range of 40-50%.

Experimental

Materials Acrylonitrile (AN, A.R. grade, Tianjin Kermel Chemical Reagents Co., Tianjin, China) was freshly distilled under vacuum. Potassium peroxydisulphate (KPS, A.R. grade, Tianjin Reagent Chemical Co., Tianjin, China) was used as an initiator. Itaconic acid (IA, C.P. grade, Shanghai Yingyuanka Chemical Co., Shanghai, China) was recrystallized from acetone to be used as a comonomer. Poly(vinyl alcohol) (PVA, Shanghai Aibi Chemical Co., polymerization degree=1750±50, Shanghai, China) was used as the dispersant. Span60 (A.R. grade, Tianjin Fuchen Chemical Reagents Factory, Tianjin, China) was used as the emulsifier. Deionized water was adopted as the polymerization medium.

Polymerization All ingredients used are summarized in Tab.1. AN/IA copolymerization was performed using a 100 ml round bottomed flask equipped with a stirrer (double- bladed, Teflon) under N2 atmosphere. A typical procedure was as follows: required amounts of KPS, IA, deionized water, dispersant, emulsifier and freshly distilled AN were injected into the reaction system in that order. The mixture was stirred at room temperature until all solid dissolved. Then the flask was wholly immersed in an oil bath held at 70 ºC by a thermostat to start the polymerization. The reaction was carried out under stirring for a definite time. The polymerization was terminated by cooling the flask in ice water. The polymerization product was obtained after filtration, washing and drying.

Tab. 1. Standard recipe for the suspension emulsion polymerization of AN with IA.

Ingredient Dosage(g) AN 28.21 H2O 9.00 KPS 0.21 IA 1.40 PVA 0.05 Span60 0.19 The conditions were as follows: 70 ˚C, 3 h, the agitation rate was 500 rpm, mass ration of water/monomer was 0.3:1 and the weight of AN was kept at 28.21 g. The concentrations of KPS (0.74 wt%), IA (5.0 wt%), PVA (0.18 wt%) and Span60 (0.67 wt%) were all based on the weight of AN.

Characterization The volume particle size (Dv) and particle size distribution of AN/IA copolymers particles were measured with a laser particle size analyzer (LS13320, Beckman Coulter, Los Angeles, CA). The samples were dispersed in ethanol under ultrasonic vibrations before measurement.

9 The particle morphology of AN/IA copolymers was observed with scanning electron microscope (SEM; JSM-5610LV, JEOL, Tokyo, Japan), and the samples were sputter-coated with Au film before the examination. Conversions were determined with the conventional gravimetric method. The conversion was calculated as follows:

Cv=M/(MAN+ MIA) where Cv is the calculated conversion, M is the weight of AN/IA copolymers, MAN is the weight of monomer AN and MIA is the weight of comonomer IA.

Acknowledgements The authors are grateful for the financial support by the Natural Science Foundation of Shandong Province (Nos. Q2006F05, Y2005F11), and the Natural Science Foundation of Ludong University (Nos. L20062901, 032912, 20052901).

References [1] Zhang, W. X.; Li, M. S.; Wang, C. G.; Zhu, B. Polym. Bull. 2002, 49. [2] Guo, J. T.; Zhang, H. X.; Wang, B. J. Funct. Mate. 2007, 38, 279. [3] Chen, X. P.; Weng, Z. X. Chem. Prod. Technol. 2000, 7, 17. [4] Bajaj, P.; Sreekumar, T. V.; Sen, K. Polymer 2001, 42, 1707. [5] Bahrami, S. H.; Bajaj, P.; Sen, K. J. Appl. Polym. Sci. 2003, 89, 1825. [6] Ge, H. Y.; Liu, H. S.; Chen, J.; Wang, C. G. J. Appl. Polym. Sci. 2008, 108, 947. [7] Devasia, R.; Reghunadhan Nair, C. P.; Ninan, K. N. Polym. Advan. Technol. in press. [8] Devasia, R.; Reghunadhan Nair, C. P.; Sadhana, R.; Babu, N. S.; Ninan, K. N. J. Appl. Polym. Sci. 2006, 100, 3055. [9] Devasia, R.; Reghunadhan Nair, C. P.; Sivadasan, P.; Ninan, K. N. Polym. Int. 2005, 54, 1110. [10] Devasia, R.; Reghunadhan Nair, C. P.; Ninan, K. N. Polym. Int. 2003, 52, 1519. [11] Mahdavian, A. R.; Abdollahi, M. J. Appl. Polym. Sci. 2007, 103, 3253. [12] Devasia, R.; Reghunadhan Nair, C. P.; Ninan, K. N. Polym. Int. 2005, 54, 381. [13] Kim, J. W.; Joe, Y. G.; Suh, K. D. Colloid. Polym. Sci. 1999, 277, 252. [14] Al-Harthi, M.; Sardashti, A.; Soares, J. B. P.; Simon L, C. Polymer, 2007, 48, 1954. [15] Kumar, A.; Prasad, B.; Mishra, I. M. J. Hazard. Mater. 2008, 150, 174. [16] Huang, X. J.; Yu, A. G.; Xu, Z. K. Bioresource. Technol. 2008, 99, 5456. [17] Tripathi, G.; Srivastava, D. Mat. Sci. Eng. A: Struct. 2007, 443, 262. [18] Wang, Y. Z.; Zhu, B.; Wang, Y. X.; Cai, H. S. New. Carbon. Mater. 2001, 16, 12. [19] Chen, H.; Ji, C. N.; Qu, R. J.; Wang, C. H.; Wang, C. G. Eur. Polym. J. 2006, 42, 1093. [20] Vindeboghel, P.; Nogues, P.; Guyot, A. J. Appl. Polym. Sci. 1994, 52, 1879. [21] Vindevoghel, P.; Nogues, P.; Guyot, A. Appl. Polym. Sci. 1994, 52, 1879. [22] Bao, Y. Z.; Wei, Z. L.; Weng, Z. X.; Huang, Z. M. Chinese. J. Polym. Sci. 2003, 21, 447. [23] Bao, Y. Z.; Wei, Z. L.; Weng, Z. X.; Huang, Z. M. Chinese. J. Chem. Eng. 2003, 11, 431. [24] Bao, Y. Z.; Wang, C. X.; Huang, Z. M.; Weng, Z. X. Chinese. J. Polym. Sci. 2004, 22, 543.